The invention relates generally to the field of lighting systems and, more particularly, to high-intensity discharge (HID) lamps. Specifically, embodiments of the present technique provide improved sealing features for such lamps.
High-intensity discharge lamps are often formed from a tubular body or arc tube that is sealed to one or more end structures. The tubular body may be made of any ceramic material, including polycrystalline alumina (PCA), sapphire, single crystal yttria aluminum garnet (YAG) and polycrystalline YAG. The end structures are often sealed to this ceramic tubular body using a seal glass, which has physical and mechanical properties matching those of the ceramic components and the end structures. Sealing usually involves heating the assembly of the ceramic tubular body, the end structures and the seal glass, to induce melting of the seal glass and a reaction with the ceramic bodies to form a strong chemical and physical bond. The ceramic tubular body and the end structures are often made of the same material. However, certain applications may require the use of different materials for the ceramic tubular body and the end structures. In either case, various stresses may arise due to the sealing process, the interface between the joined components, and the materials used for the different components. For example, the component materials may have different mechanical and physical properties, such as different coefficients of thermal expansion (CTE), which can lead to residual stresses and sealing cracks. These potential stresses and sealing cracks are particularly problematic for high-pressure lamps.
Additionally, the geometry of the interface between the ceramic tubular body and the end structures also may attribute to the foregoing stresses. For example, the end structures are often shaped as a plug or a pocket, which interfaces both the flat and cylindrical surfaces of the ceramic tubular body. If the components have different coefficients of thermal expansion and elastic properties, then residual stresses arise because of the different strains that prevent relaxation of the materials to stress-free states. For example in the case of the plug type end structure, if the plug has a lower coefficient of thermal expansion than the ceramic tubular body and seal glass, then compressive stresses arise in the ceramic-seal glass region while tensile stresses arise in the plug region.
Typically, the seal glasses used for sealing ceramic lamp components are required to be non-reactive with the different species in the lamp environment and to possess microstructural stability during the life of the lamp, in addition to having a melting temperature and a crystallization temperature above the lamp operating temperature. However, for high temperature lamp applications, these are challenging requirements.
In sealing techniques used currently, the seal glass is generally melted using a furnace cycle, such as a large muffle type furnace, with temperatures up to 1750 degrees centigrade. The seal glass and the ceramic components to be sealed are inserted into a base of the furnace and the furnace is operated through a controlled temperature cycle. The controlled temperature cycle is designed in conjunction with a temperature gradient at the end of the furnace to melt the seal glass (typically a dysprosia-alumina-silica mixture), which then flows through the gap between components to be sealed. The seal length is controlled in part by the temperature gradient.
The above approach may be disadvantageous in several respects. Firstly, the furnace used in the above described technique provides a relatively diffuse heat source with a low temperature gradient. Hence, the technique does not provide a desired control of seal length and seal microstructure. Further, the above technique may not be useful for a wide variety of lamp geometries. For example, in lamps having a short aspect ratio (i.e., ratio of length to diameter), an important consideration while sealing an end of the lamp is to preserve the dosing material inside the lamp. The above technique may prove disadvantageous in such applications, as the diffuse heat produced by the furnace may result in undesirable heating of the dosing material during sealing of one end of the lamp.
Accordingly, a technique is needed to provide a lighting system with improved sealing characteristics for sealing a wide variety of ceramic lamp components having varied geometries.